Filtering processes are widely used to remove a variety of contaminants from a gaseous or liquid fluid stream. In a filtering process, the fluid is passed, either singly or repeatedly, through a filter medium; therefore, filter throughput is effectively limited by the surface area of the medium, and high capacity filters include large area bodies of filter medium. In order to minimize the size of filter assemblies, a variety of filter configurations have been developed. The choice of configuration will depend upon the nature of the filter medium itself as well as the process requirements of the filtration, for example, volume being filtered, pressure drop, the nature of the contaminants and so forth.
One particular type of filter configuration includes a spirally wound body of filter medium disposed so that the fluid stream will either pass axially or radially therethrough. Another type of configuration comprises a pleated medium. One specific configuration of pleated medium comprises a relatively planar medium, formed into a cylinder and pleated longitudinally. In some instances, the pleats are spirally wrapped about the axis of the cylinder. Such spirally wrapped, pleated configurations are occasionally referred to as semi-spirally wound filters. U.S. Pat. Nos. 3,042,571 and 2,1322,548 disclose various configurations of pleated, and spirally wound filters.
One particularly important class of filters includes porous membrane filter elements. These membranes are typically fabricated from polymeric materials and include pores having precisely controlled sizes ranging from macroporous through microporous to ultraporous. Such membranes are typically fairly thin and fragile, and filters including porous membranes usually incorporate a support element. U.S. Pat. No. 4,500,426 discloses a number of configurations of filters including porous membranes, and FIGS. 13-18 disclose a pleated, spirally wound membrane filter. As shown therein, the membrane is folded about a spacer and wound about a central core. The central core is necessary to provide support and stability to the filter element so as to prevent damage to the relatively fragile membrane. It is desirable to eliminate the central core for a number of reasons. The core limits the active area of the filter to some degree, and the use of a core increases the material cost of the filter element as well as the time and labor required for its manufacture; however, prior art membrane filters have not had sufficient structural integrity to permit their manufacture without a central support core. The present invention provides a unique configuration of pleated, spirally wound filter which incorporates a porous membrane as the active filter element thereof. The filter of the present invention is a self-supporting, coreless structure. It is easy to fabricate and highly efficient in both its use of materials and its operation. These and other advantages of the present invention will be readily apparent from the drawings, discussion and description which follow.